Blow-by gas leaking through a gap of a piston ring into a crankcase during engine compression and explosion strokes, which fills the crankcase and a cylinder head cover in communication therewith, is required to be released outside. However, the crankcase and the cylinder head cover are full of oil mist since the crankcase has therein a crankshaft, a connecting rod and the like which move fast, and the cylinder head cover in communication with the crankcase has therein a rocker arm, a valve and the like which are in work.
Thus, release of the blow-by gas as it is to the atmosphere may disadvantageously cause the oil mist mixed in the blow-by gas to be also discharged outside, so that provided is a closed breather (CCV: Closed Crankcase Ventilator) having therein a filtering net or a labyrinth structure for separation and recovery of the oil mist in the blow-by gas. The blow-by gas extracted from the cylinder head cover of the engine is passed to the closed breather for separation and recovery of the oil mist and is returned through a gas return pipe to an intake pipe while the oil recovered by the closed breather is returned through an oil recovery pipe to an oil pan (not shown).
In such closed breather system, when the cylinder head cover is dismounted, for example, for maintenance of the engine, the gas return pipe is also removed from the intake pipe. After completion of the maintenance or the like, there is a fear that only the cylinder head cover is remounted while re-connection of the gas return pipe to the intake pipe may be forgotten. Thus, obligated in U.S.A. from 2010 onward is to detect disconnection (forgotten connection) of the gas return pipe in a larger vehicle.
In case of the forgotten connection of the return pipe, air may be taken in also through a portion of the intake pipe to which the gas return pipe is to be connected; then, even if a flow rate of air fed into the engine remained unchanged, an air flow rate (mass flow rate of air) detected by an air flow sensor would be reduced. Thus, for example, Patent Literature 1 mentioned hereinafter has suggested a technique that a normal air flow rate stored, for example, at a start of the engine in a last drive cycle (from key-on to key-off) is compared with a value detected by the air flow sensor in a current drive cycle under operational conditions unchanged to determine disconnection of the gas return pipe when the detected value is lower than the normal air flow rate by a predetermined threshold value or more.